This invention relates to image sensors and more particularly relates to methods for controlling the dynamic range of an image sensor.
CMOS image sensors are increasingly becoming the preferable selection over CCD image sensors for a wide range of solid-state imaging applications. This is particularly true for those applications that demand the inherent advantages of CMOS fabrication such as low cost, small feature size production, and large-scale monolithic integration. But because the illumination range, or dynamic range, characteristic of CCD image sensors is generally larger than that of CMOS image sensors, for many applications CMOS image sensor performance is suboptimal to CCD image sensor performance. The reduced dynamic range of CMOS imagers is generally due to a readout noise level that is higher than that of CCD imagers. This performance limitation of CMOS imagers results in a required trade-off between operational and functional features together desired of an image sensor for a given application.
For many challenging applications like machine vision applications, the combined performance, operational, and functional capabilities of CCD and CMOS image sensors are desired, however. Specifically, the low cost and small size that can be achieved for a typical CMOS imager is often desired in combination with the performance advantages of a CCD imager. In a typical machine vision application, high illumination resolution is required, to enable object comparison based on object surface characteristics such as texture, color, or orientation, all of which may be manifested as subtle changes in light intensity distribution. High spatial resolution is also required, to enable a sufficiently wide field of view and to enable high precision of image processing tasks such as edge detection, median filtering, smoothing, and segmentation. Image sensing capability at a high frame rate is often required to avoid loss of visual information, especially for applications such as intelligent transportation systems where the imager or the environment to be imaged may be moving at high speed. A short image integration time is also here desired for minimization of captured image blurriness caused by mechanical vibrations of the moving sensor.
A particularly stringent requirement for many machine vision applications is a wide illumination dynamic range. Such is typically required to enable effective imaging of natural scenes; a naturally lit scene may have a dynamic range that is four orders of magnitude or more. For applications such as vehicle-based imaging systems that inherently have no control over natural or artificial light in a scene to be imaged, the dynamic range of an imager can be the limiting performance characteristic of the imager.
There have been made many diverse proposals for enabling an increase in the dynamic range of an image sensor in general. Such proposals range from changes in design of the photodiodes employed in imager pixels, to modifications in the conversion gain of imager pixels, to pixel-level circuit configurations, to customized system-level architecture and control configurations. In general such techniques are found to provide an inadequate degree of dynamic range expansion, to require a tradeoff with spatial resolution, frame rate, size, cost or other image sensor consideration, or to impose excessive operational requirements. As a result, the multifaceted demands of many imager applications have generally not been fully addressed in a single imager system.